To further aid my own work in audio amplifiers I needed a good quality audio oscillator. In particular I wanted a low distortion source so that when testing amplifiers I could get some idea of the distortion produced by the amplifier. I also wanted a source that could have sufficient drive capability to directly drive a loudspeaker. Most bench top generators have a 600 Ohm output which cannot be used to directly drive a speaker.

Specification

10Hz – 100kHz in decade stages

0-5V RMS output

Capable of driving into 8 Ohms

Lowest distortion levels achievable.

Description

I began this project in 1992 during my first year at University. I used the basic Wein Bridge oscillator as the basis for the generator. I opted to use the then industry standard LM318 op-amp for the oscillator. This op-amp had better specs than the common 741 and also proved to deliver lower distortion figures. A Wein Bridge oscillator requires an amplifier circuit with a precise voltage gain of 3. If the gain is greater than 3 then the oscillations build up until the amplifier saturates producing considerable harmonics. If the gain is less then 3 the oscillations die away. So the trick with getting really low distortion from a Wein Bridge oscillator is to stabilise the gain to 3 within the linear range of the amplifier. This can be done with some sort of temperature dependant resistor in the negative feedback loop – possibly a thermistor or even a small light bulb. Since I was unable to obtain a suitable thermistor I opted for the light bulb approach and experimented with a variety of different small bulbs, such as 3V, 6V, 12V until I found one that achieved the right resistance and right temperature dependence to stabilise the circuit.

The output of the oscillator was then fed into another amplifier circuit which buffered it from the load. The amplifier circuit consisted of another LM318 driving a discrete class AB push-pull transistor pair, thus giving it enough drive capability to drive a loudspeaker.

I designed the circuit board layout again on paper as I had done with the power supply and etched the PC board in a similar fashion. This I mounted into an aluminium box I had made and wired it up.

Internal view of the Audio Oscillator

When it came to setting up the oscillator gain for lowest distortion, I constructed a notch filter on bread board and placed variable resistors in the tuning elements for ultra-fine tuning and adjustment. I would then listen to the notch filter output signal through headphones and adjust the tuning until the fundamental signal of the oscillator vanished. I discovered that the best frequency for these tests was around 400Hz. At this frequency the human ear is probably most capable of discerning the sound of a 400Hz tone from its 800 or 1200Hz second and third harmonics. So with the oscillator output set to Maximum (close to 5V RMS) I would adjust the oscillator frequency to the centre frequency of the notch filter and then fine tune the notch to completely eliminate the 400Hz tone. Obviously without the notch filter a 5V RMS signal driving directly into headphones would be deafening. As I fine tuned the notch I monitored the residual output with a multimeter. I was able to adjust the filter such that the residual output came down to a few millivolts. Even a few millivolts signal driven into headphones is still quite audible.

I then adjusted the feedback drive level to the oscillator to the point where I could no longer hear the second or third harmonic, at this point the multimeter gave its lowest reading which I noted. I then measured the raw output from the oscillator to obtain the reference level. Using this method I was able to determine that the oscillator was giving me harmonics around -80dBc – a fairly impressive figure for a simple analogue generator. I was able to verify these measurements a few years later using the audio signal analyser at my university (see plots below). My test method may seem crude, however it delivered a surprisingly accurate result. It is in effect the basic circuit that is used in audio distortion meters.

This test method however was only able to characterise the oscillators performance at 400Hz and there are 4 decades of frequencies it operates over. The oscillators distortion performance tends to drop off at low frequencies (near 10Hz), this is due to the thermal time constant of the light bulb being fast enough for the filament resistance to change during a single sine wave cycle thus effecting the amplifier gain. This will obviously manifest itself as increased distortion. Likewise at high frequencies (10kHz – 100kHz) the performance degrades. Here again the oscillator's op-amp open loop gain will be considerably reduced and thus its distortion figures will be worse. Other factors which affect distortion will be the choice of capacitors used in the Wein Bridge – ideally these should be the most stable type with lowest loss tangent. In addition the matching between the pairs of capacitors and resistors is not perfect and as one switched from one range to the next, the capacitor matching changes.

On the whole however this design has proven itself to be quite adequate for developing and testing audio amplifiers and I still use it today when I need a good clean audio source.